Various methods of energy storage have been investigated in order to help integrate intermittent or invariant generating methods into electricity supply grids. Wind energy in particular, can require substantial backup and storage technologies to facilitate widespread use. Even with a relatively small proportion of wind energy, some capacity will be wasted when demand is low, and there are corresponding periods of higher demand but insufficient supply where backup generation must be used. The situation can be exacerbated as the proportion of wind energy increases. Supply and demand mismatch with wind can occur seasonally as well as in daily cycles, which creates a particular opportunity for storage systems with large generation duration times. Many large scale grid energy storage systems have found it difficult to compete with conventional gas turbines for load levelling variable energy sources, partly due to high capital costs, a shortage of potential sites, long build times, and energy losses due to various system inefficiencies.
Compressed air energy storage is well established in prior art. Such systems use air that has been compressed and stored during off peak periods to generate electricity on peak. The energy content of a quantity of compressed air is determined both by its pressure and its temperature, which temperature will increase with pressure. Adiabatic storage methods attempt to retain the heat of compression for recovery on expansion to increase efficiency levels, whereas simpler diabatic methods have no mechanism for retaining this heat. Storing compressed air in large underground formations, within pressure vessels, and under hydrostatic pressure is prior art. Methods of increasing power output by pre-heating the air with a waste heat source at a useable temperature, or by removing, storing, and then returning the heat of compression have also been investigated. The comparatively rapid response times possible with compressed air energy storage is particularly relevant to its ability to provide a backup generation source for wind.
The most common methods of electrical generation from a thermal energy source use turbo machinery to extract mechanical work, which mechanical work is used to drive a generator. The most common turbine cycles are the Brayton, Rankine, and combined cycles. The turbine's working fluid remains in gaseous form throughout in the Brayton cycle, where it is first compressed, then provided with a heat source (usually combustion), and then expanded through a turbine to recover energy. The working fluid is not usually re-circulated within the Brayton cycle although such closed cycles would still fit within the definition. In contrast, the Rankine cycle continuously re-circulates its working fluid, which is present in both liquid and gaseous form at different stages in the cycle. The fluid in gaseous form, which has been expanded through the turbine to extract work, is condensed back to liquid to create a vacuum and flow in the turbine. That condensed liquid is then extracted from the condenser, re-pressurised, and introduced to a heat source where it is vaporised and supplied back to the turbine in gaseous form. The working fluid to be condensed is typically steam, and the fluid used to condense is typically air or water. Typical pressures within a steam condenser are sub-atmospheric at around 0.05 bar (5000 Pa). The significant amount of waste heat from condensing is dispersed as the temperatures involved are too low to be practicable for further energy recovery. The efficiency levels in terms of electrical recovery are up to 40% for both cycles. Combined cycle arrangements use both the Brayton and Rankine cycles, where the Rankine cycle extracts heat from the exhaust of the Brayton cycle to achieve an aggregate 60% electrical efficiency levels.
Hydrogen combusting gas turbines are also prior art. These turbines may be air breathing and produce the pollutant NOx, or combust hydrogen and oxygen gas in stoichiometric ratios, producing only steam. By way of example, a recuperating hydrogen oxygen combusting gas turbine has been disclosed in US Pat No WO97/31184 issued to Westinghouse Electric Corporation, where the waste heat from the steam is recuperated into the hydrogen fuel and oxygen. An energy storage method using a hydrogen oxygen combusting gas turbine with submerged water electrolysis and hydrostatically pressurised fuel and oxidiser storage is disclosed in French Pat No FR2286891 issued to Imberteche in 1976, which system does not specify a method of recovering the latent heat of vaporisation of the steam.
A peaking power system of an air breathing gas turbine using a compressed air storage system is disclosed by Flynt, in U.S. Pat. No. 3,831,373 published in 1974. The gas turbine disclosed can either operate conventionally, or the compressor of that turbine can be powered by off-peak electricity and used to compress air for storage, and on peak, the stored air can be released through the combustor and turbine in place of the compressor for increased generation output. Because the gas turbine is air breathing, its components can in effect be used simultaneously as part of the compressed air system. The air is stored under hydrostatic pressure in this system. In one embodiment, the system includes a method of using the heat of compression produced during storage by using a flow of water in a heat exchanger to produce steam, which steam is then expanded through the turbine and the rotational energy used to supplement the compressor.
The method of using the Venturi effect for both cooling and heating has been disclosed in U.S. Pat. No. 3,200,607 issued to Williams in 1965 whose space conditioning apparatus can be operated to provide either cooling or heating, and U.S. Pat. No. 2,441,279 issued to McCollum in 1942 whose Venturi system can simultaneously be used for cooling aircraft components and the heat extracted can be used for air conditioning. The method of using the Venturi effect within a heat exchanger to exchange heat between two mass flows is also disclosed in Patent Specification GB1,419,490 by Cowans in 1971. A further description of heat transfer using the Venturi effect is described in US Patent Application US2009/0223650 filed by Williams, which considers the possibility of using heat from a Venturi heat exchanger for power generation without elaborating as to any methodology. Although that document discusses exploiting the thermodynamic phase changes by the Bernoulli heat pump and notes the high energy content available due to such a phase change, that document does not disclose any mechanism or methodology for utilising that phase change.
According to the present invention there is provided a power generating system comprising a thermal power plant including:                (a) a vaporiser means for vaporising a first working fluid, a conduit means for conducting said (vaporised) first working fluid to a main power generating turbine for extracting energy from the first working fluid;        (b) conduit means for taking first working fluid exiting the main power turbine to a Venturi condenser, the first working fluid passing through heat exchanger means in the Venturi condenser to transfer heat to a second working fluid;        (c) the Venturi condenser, provided with an inlet for receiving a second working fluid at elevated pressure, an inlet portion leading to one or more Venturi tubes, the Venturi tubes having at least one converging inlet portion, at least one a straight constricted portion and at least one diverging outlet portion, and heat exchanger means surrounding the outlet portion;        (d) a second power turbine for extracting energy from the second working fluid exiting the one or more Venturi tubes;        (e) conduit means for returning said first working fluid to the vaporising means;        (f) pump means for pressurising said first working fluid and returning said first working fluid to the vaporising means;        (g) pump means for optionally pumping the second working fluid to a hydrostatically pressurised storage unit;        (h) storage means for storing said second working fluid in gaseous state under hydrostatic pressure;        (i) conduit means for conducting the second working fluid from said storage means to the inlet of the Venturi condenser;        (j) control means for controlling operation of the system.        
Advantageously, the use of the Venturi condenser operating at an elevated pressure allows an increased efficiency of operation and allows the more effective cooling of a first working fluid exiting a main power generating turbine. The cooling effect on the fluid passing through the Venturi tube results in a greater temperature difference across the heat exchanger in the Venturi condenser than might otherwise be possible. This more effective cooling reduces the pressure of the fluid exiting the condenser and so more effectively draws the first working fluid through the main power generating turbine. Additionally, the energy transferred to the second working fluid in the condenser is sufficient to allow worthwhile and beneficial energy extraction by the secondary turbine, so increasing the overall system efficiency.
According to another aspect of the present invention there is provided a method of energy recovery for a thermal power plant which:                (a) a first working fluid delivering energy to a main power generating turbine then passes through a heat exchanger means in a Venturi condenser whereupon at least some of the remaining energy is extracted, and at least some of the first working fluid condenses to a liquid state        (b) a second working fluid enters one or more Venturi tubes in a Venturi condenser at elevated pressure, the second working fluid cooling and decreasing in pressure as it passes through the Venturi tubes;        (c) the second working fluid absorbing thermal energy from the first working fluid in a heat exchanger means in the Venturi condenser;        (d) the reduced volume of the first working fluid causing a decreased pressure downstream of the main power generating turbine so increasing flow of the first working fluid through the main power generating turbine;        (e) the second working fluid after being heated in the heat exchanger means passing through a second power generating turbine where energy is extracted.        
A particular embodiment of the system could be based around a 500 MW output hydrogen oxygen combusting gas turbine in a combined cycle arrangement, a water electrolysis system to supply the gasses for combustion, and a compressed air energy and Venturi condensing system which extracts the heat of vaporisation from the turbine cycle. The hydrogen oxygen turbine might be expected to achieve efficiency of around 62 percent, requiring around 804 MW combustion of hydrogen to produce that output. Around 10 percent of the combustion energy will be lost to component inefficiency, but around 266 MW will be lost due to the latent heat of vaporisation of the steam, which energy is not usually recoverable due to the low temperatures involved. The fuel and oxidiser requirement of such a turbine would be around 5.6 kg/sec of hydrogen and 45.4 kg/sec of oxygen to produce 51 kg/sec of superheated steam.
The conversion efficiency at producing hydrogen and oxygen gas for modern electrolysers is high, around 90-95% or more. Where the gasses need to be compressed or liquefied for storage, the chemical energy remaining will be around 65-70% percent of what. With a hydrostatically pressurised water feed and hydrostatic storage, the gasses do not need to be compressed, as the pressure head differential provides both compression and gas transmission forces. Such a system would be gravity fed and need no fuel and oxidiser pump. Additional thermal energy will be conserved by supplying the gasses at ambient temperatures rather than cryogenic temperatures, which is especially relevant given the very high specific heat capacity of hydrogen. Assuming an electrolyser efficiency level of 95%, the energy requirement for the electrolyser would be 845 MW. Proton exchange membrane electrolysers are capable of handling partial loads without compromising efficiency and can reach peak operating conditions rapidly, making them desirable for integrating intermittent energy sources and accommodating stochastic variations.
This electrolysis and hydrogen oxygen turbine based system would typically be combined with a compressed air system. Operating on its own, compressed air energy storage system might return round trip electrical efficiency levels of around 60 to 70 percent for isothermal systems where the heat of compression is reused, or 55 percent where the heat is dissipated. A Venturi condenser powered by hydrostatic pressure is used to remove and recover the heat of vaporisation energy most efficiently. It can be assumed that the air will be supplied to the Venturi at around between 4 and 25° Centigrade depending on the ambient conditions. The temperature in a deep coal mine will be significantly warmer than seabed temperatures. An air cooled condenser without the Venturi effect would only be able to condense due to the modest temperature difference between the steam entering the condenser and the temperature of the air used to extract heat. Using the Venturi effect to reduce temperature allows more intensive energy removal and recovery. For an observable temperature drop, it is necessary to increase the velocity to above Mach 0.3, otherwise the compression effects will be negligible. At high subsonic velocities of around Mach 0.8, the temperature is likely to drop to around −30 degrees Centigrade, and the air mass flow required to absorb the 266 MW latent heat of vaporisation would then be around 4140 kg/sec. Although passing through the transonic region creates complications, supersonic Venturi flows are achievable. Velocities of around Mach 2 will reduce absolute temperature to around 40% of the original temperature, or −151 degrees Centigrade. The air mass flow required in such an embodiment would be around 1445 kg/sec. For larger overall depressurisations, it may be preferable to depressurise in several stages and also recover energy during the intermediate stages to avoid structural problems and ice formation, and for larger scale configurations involving a substantial air mass flow, parallel Venturi effect flows are likely to be preferred to maximise surface area, reduce wall thicknesses, and optimise flow.
Bernoulli's principle concerns the equivalence of static and dynamic pressure in fluid flow. As a pressurised fluid is released and gains velocity, some of the static pressure or potential energy of that fluid is converted into dynamic pressure or kinetic energy. The total pressure, which is the sum of the static and dynamic pressures, remains constant in absence of any external factors. However, in the present system, thermal energy in the Venturi condenser is an external factor which causes the air volume to expand. In the downstream direction towards the air motor or turbine, this expansion causes an increase in dynamic and therefore total pressure, which allows a more energetic expansion. In the upstream direction towards the storage unit, this thermal expansion is against the direction of flow, therefore this backpressure converts both itself and also some of the velocity of the air into static pressure. As before, the upstream gas also has an increased total pressure, although unlike the downstream flow, the velocity would reduce rather than increase. This effect continues into the hydrostatic storage unit to where the velocity is zero. Since there is no dynamic pressure at that point, the static pressure of the air momentarily increases above the hydrostatic pressure level. This additional pressurisation energy combined with the hydrostatic pressurisation is instantly available to drive the gas through the riser pipe-work and Venturi.
The electrical efficiency level of the electrolysis and hydrogen oxygen combusting turbine described above will be around 60 percent in isolation without recovering the heat of vaporisation, and similar efficiency levels can be expected of the compressed air subsystem. By combining the two systems, the combustion turbine no longer needs to pump significant quantities of water through that condenser to remove the heat since the Venturi condenser now performs that function. In addition, around 90 percent of the latent heat of vaporisation energy from the turbine is now recoverable in the Venturi condenser. The combined efficiency levels of the systems operating together are likely to be in the region of 80 percent or more.
In terms of energy density, at normal atmospheric pressure, hydrogen has a volume energy density of 3 kWh/cubic meter, and the amount of hydrogen produced from water electrolysis would be around twice the volume of oxygen. At 500 meters, the volume of both gasses reduce to less than 2 percent of the surface volume, giving volume energy densities of the hydrogen and oxygen gasses of 246 kWh per cubic meter. Such depths are commonplace within existing deep coalmines, many of which are now disused. The amount of air required to absorb the vaporisation energy as a ratio of the hydrogen and oxygen volumes is estimated at up to 100 times the volume for the subsonic case, and up to 10 times in the Mach 2 supersonic case. Even higher velocities might be practicable, which would potentially further reduce air volumes and condenser sizes, and further increase efficiency levels. These volumes compare favourably with hydroelectric pumped storage, where each cubic meter of water stores around 1-1.5 kWh of energy, and even more favourably to compressed air storage. Due to the very different energy densities involved, a system which displaces the equivalent water volumes of a hydroelectric pump storage plant with 6 hours generation duration might now be capable of powering the grid continuously for 4 consecutive days or more with a comparable instantaneous power output. The marginal costs of increasing the power capacity, say by adding an additional 1 GWh of storage, would be a small fraction of the pumped storage or compressed air energy storage equivalent.